338 ZOOLOGY. 



ing to tull the tale of degeneracy and descent from a 

 quadrupedal terrestrial ancestor. 



On the other hand the increase in the length of the 

 linihs of the horse is dne to use, to impact and longitudinal 

 strains. Eeduction in the number of digits, as in oxen, 

 deer, and the horse, was supposed by Ryder to be due to the 

 elongation of those which receive the greater number of 

 strains and impacts in rapid progi'ession, and the comple- 

 mentary loss of material available for the growth of those 

 parts not subject to this stimulus. In jerboas and 

 kangaroos, which use the hind limbs in leaping, these only 

 have reduced digits, the fore limbs retaining their prim- 

 itive five-toed character. The modifications of the fore feet 

 of moles (Figs. 298 and 300), Echidna, the ant-eaters 

 (Cholajpas, etc.), the mole cricket, and of the digging 

 scaraba?id beetles are cases of convergence, the same class 

 of strains and other niovenients in animals of widely different 

 classes bringing about identity in form, ultimately due to 

 change in habits. 



The same law holds in the case of the teeth of mammals. 

 Increase in size of a tooth or part of a tooth is due to 

 increased use. Cope claims that the form of canine teeth 

 is due to strains sustained by them on account of their 

 position in the jaws at points which are naturally utilized 

 in the seizing of prey or when fighting with their enemies. 

 So also the horns of reptiles, of mammals, of caterpillars 

 and other insects, are the result of occasional or inter- 

 mittent blows or stimuli in repelling or making attacks. 



Other Factors. — It is by the action of agencies such as 

 we have specified that variations have arisen, and it is a 

 matter of fact tliat variations occur in fossil shells in the 

 earliest as well as later geological periods. 



Indeed, the strongest, most convincing proof of the theory 

 of evolution is derived from the facts of palaeontology, the 

 succession of extinct forms being, as a rule, from the gen- 

 eralized to the specialized; also the earliest forms were 

 more plastic, varied more rapidly, and thus led to the origin 



